IMAP Timeouts

I am running a Zimbra Network Edition 5.0.18 server. It is running on top of RHEL 4. It has, exactly, 500 users right now and there are usually around 200 connections at any given time.

Ever since we hit 400, or so, users we have been experiencing timeouts with IMAP connections. There is no real rhyme or reason and it's not always the same people. Sometimes it happens with users on the same network as the server and sometimes it happens with users that are hitting the server from afar. The one thing that is consistent is that it happens during peak hours.

Using MRTG I've noticed that the NIC is not taxed. Average throughput is 24kBps In and 58kBps Out. Max is 848kBps and 2,995kBps. This doesn't seem crazy considering we have a 20/20Mbps Internet connection. Duplex is set to full on the interface and all that jazz seems okay.

I'm really at a loss. Does anyone have any clue as to why I would be having intermittent timeouts on port 143? Is it possible that an upgrade to 5.0.19+ would help or even an upgrade to 6? Thanks.